The invention also relates to an assigned electrooptical assembly for receiving optical radiation and generating a resultant electrical signal for an electrooptical distance measuring instrument of the generic type.
Moreover, the invention relates to an electrooptical distance measuring instrument and distance measuring method using the detector according to the invention and the assigned electrooptical assembly.
For measuring a target point, a multiplicity of geodetic methods and geodetic instruments have been known since ancient times. In this case, distance and angle from a measuring instrument to a target point to be measured are recorded and, in particular, the location of the measuring instrument together with reference points possibly present are detected as spatial standard data.
Such measuring instruments are used for detecting three-dimensional objects or surfaces. In this case, these instruments typically progressively scan a three-dimensional structure, such as e.g. the structure of a construction site, using laser pulses and then calculate from the reflection pulses regained a corresponding three-dimensional model that describes the surface of the object.
One generally known example of such measuring instruments or geodetic instruments is a theodolite, a tachymeter or a total station, which is also designated as electronic tachymeter or computer tachymeter. One such geodetic measuring apparatus from the prior art is described in the publication document EP 1 686 350, for example. Such instruments have electrical-sensor-based angle and distance measuring functions that permit direction and distance to be determined with respect to a selected target. In this case, the angle and distance variables are ascertained in the internal reference system of the instrument and, if appropriate, also have to be combined with an external reference system for absolute position determination.
Apparatuses for optically scanning an environment that are embodied as laser scanners usually comprise a measuring head mounted on a base, said measuring head being rotatable relative to said base about a base rotation axis. In the measuring head, there are accommodated on one side a laser light source and a light sensor for the reception of reflected laser pulses, and also a transmission and reception optical unit and an exit opening that permits the radiation to emerge from or enter the housing. On the other side of the measuring head, opposite the exit opening, the measuring head has a rotary mirror for deflecting transmission light beam and reception light beam, said rotary mirror being rotatable about a rotation axis that is perpendicular to the base axis. The intersection point of the two rotation axes generally corresponds to the point of impingement of the transmission light beam on the rotary mirror, wherein the rotary mirror is generally arranged in a manner inclined by an angle of 45° with respect to the rotation axes. By rotating the measuring head about the base rotation axis and rotating the rotary mirror about the rotation axis, it is possible to carry out a three-dimensional scan.
In a different topology, the entire optical transmission and reception module is rotated instead of a rotary mirror. One example of such an arrangement is scanning theodolites.
Present-day distance measuring devices that are used in production such as theodolites, scanners, LIDAR systems (“light detection and ranging”), profilers, laser trackers, or else in automobiles, have the problem of handling a high signal dynamic range.
Furthermore, in some of these products, the sensor beam is pivoted at high speed by means of a deflection unit, in particular a scanner. In order that the reception beam reflected back from the target object impinges on the receiver, the latter has to be designed with a large field of view (FOV). Receivers having a large field of view have the disadvantage of shot rays in daylight or ambient light, however, which reduces the range and generates distance noise in distance measuring devices.
Distance measuring sensors for geodetic or industrial measuring instruments are almost exclusively equipped with an avalanche photodiode (APD) as detector having a temporal resolution in the picoseconds range. These APDs generally have a round reception area. APDs having reception areas covered with a mask, consisting of a plurality of openings, are also known.
All these avalanche photodiodes according to the prior art have a common sensitive photosensitive area situated under the mask. Just a single photocurrent is generated upon incidence of light.
APD arrays are also known. Such arrays have a matrixlike arrangement of sensor elements that are used for example for highly sensitive measurements for imaging object representation. In this case, the insensitive distance between the sensor elements is generally very large. Such arrays typically have the disadvantage of a small filling factor (<50%), and the number of pixels and corresponding signals increases with the square of the area, which makes the signal processing very complex and expensive. In conventional APDs, care is taken to ensure that crosstalk between the elements is as low as possible (typically <5%). In the case of the present invention, this requirement is not relevant to the solution of the problem addressed.
Conventional distance measuring apparatuses have to handle the high signal dynamic range. In this case, with the use of a single-area avalanche photodiode, the limits with regard to the current intensity handleable by the APD itself are reached and the reception electronics are also occasionally overdriven (supersaturation). In certain apparatuses, the emitted transmission power is adapted; in other apparatuses, the gain of the receiver is set; in still other apparatuses, a multi-channel receiver is used.
Optical fiber amplifiers connected downstream of the transmission source, for example embodied as a laser, LED, etc., are particularly suitable for setting the transmission power over a large range. However, such light amplifiers are not settable with microsecond intervals. However, present-day scanners measure at a point rate of at least one million points per second. By contrast, there are variable optical attenuators (VOA) which are settable with nanosecond speed; such components require correspondingly powerful driving and are expensive in comparison with the other customary components of a typical distance measuring instrument.
Furthermore, shadings of the reception light in the near range (e.g. <5 m) generally occur—principally in the case of receiver-end fixed-focus arrangements—in the case of known detector arrangements from the prior art. Said shadings have the effect that no signal at all is detected by the APD in a very near distance range of from 0 m to a certain limit distance. Therefore, the distance range within which targets are actually measurable or their distances are determinable is thus reduced.